The team, from the genetics unit at London's Guy's Hospital, have developed a method called pre-implantation genetic haplotyping (PGH), which they expect to offer to over 100 families a year.

The current test is known as pre-implantation genetic diagnosis (PGD).

50/50 chance

PGH involves testing parents and any existing children or relations carrying or with a genetic condition, to identify the faulty units of chromosomal DNA.

It's absolutely brilliant because it gives people in my situation more of a choice. I love my little boy, I wouldn't be without him, but it does give people the option of not having to watch their son die

Linda Ball, whose son has muscular dystrophy

Using this information, it is possible to take a cell from the embryo, treat it in the lab to create more copies of its genetic material and then look for markers that show an embryo carries two copies of these faulty units, or haplotypes.

This would mean it would be affected by the condition.

The technique has been used to test for Duchenne Muscular Dystrophy (DMD). It primarily affects boys, who inherit the disease through their mothers.

Families with a history of the condition are currently offered embryo sex testing and no male embryos are implanted, as it is not possible to tell if they have the condition - even though they have a 50/50 chance of being affected.

But with the new test, doctors are able to see if an embryo carries the tell-tale DMD haplotypes seen in its parents, meaning more embryos can be selected for use.

Genetic trail

The test also allows detection of any of the genetic mutations which can cause cystic fibrosis.

I am horrified to think of these people sitting in judgment on these embryos

Josephine QuintavalleComment on Reproductive Ethics

Like DMD, it is a recessive disease, and means both copies of chromosome 7 must carry a fault for a child to have the disease - but PGD can spot only the most common of the hundreds of faults.

The team have also helped a woman affected by hydatidiform mole - a condition where pregnancy leads to a potentially fatal tumour forming instead of a foetus.

However she said the new test would not lead to a flood of people wanting to take advantage of the science.

"Of the patients currently referred for PGD, only a third end up going through a cycle.

"These are, generally, couples who can get pregnant without having to undergo fertility treatment, and when they get to know what it involves, many do not go ahead."

Dr Mark Hamilton, chairman of the British Fertility Society, said: "Any technique which has the potential to reduce the risk of serious, debilitating and potentially life-threatening disease has to be greeted with some enthusiasm.

"We are always striving to maximise the chance that fertility treatment will be successful.

"But not transferring because we are absolutely confident they are affected by a condition, rather than because we suspect they are, is preferable and much less wasteful."

But Josephine Quintavalle, of Comment on Reproductive Ethics, warned against further extensions of screening.

She said: "I am horrified to think of these people sitting in judgment on these embryos and saying who should live and who should die."